Four-way open-center rotary control valves which use constant flow rate hydraulic power sources are commonly utilized for controlling vehicular power steering systems. Such systems typically employ a four-way open-center rotary control valve (hereinafter "rotary valve") having position feedback. Road feel is artificially induced by deflection of a torsion bar.
An earlier type of power steering system provided feedback partially proportional to actual steering effort. This power steering system featured a four-way open-center hydraulic reaction control spool valve (hereinafter "reaction valve"). However, such systems were rather complex to manufacture and were replaced by power steering systems which feature the simpler rotary valves mentioned above.
A rotary valve is a four-way open-center flow control valve which has circumferentially close fitting inner and outer valve members. The inner and outer valve members usually feature four sets each of pressure, first and second output, and return slots. These four sets of slots are equally spaced (at 90 degrees) around the interfacing circumferences of the inner and outer valve members. Differentially controlled output flows and/or pressures in the first and second output slots are obtained by rotationally displacing the inner valve member with respect to the outer valve member.
The open-center configuration of the rotary valve allows a nominally constant flow hydraulic fluid source to be utilized. In normal operation at other than small valve displacements, system supply pressure nominally approximates differential output pressure (hereinafter "output pressure"). This results in minimum system power consumption but results in wildly erratic system control characteristics wherein assist levels can vary by more than 40:1.
In accordance with the present invention, a four-way open-center rotary control valve having internally generated hydraulic reaction torque (hereinafter "torque reaction valve") is provided. In preferred embodiments of the present invention, the hydraulic reaction torque is generated between inner and outer valve members which are formed with multiple control orifices having differing radii. The control orifices comprise input control orifices which meter fluid from a constant flow hydraulic fluid source into either of first and second output ports, and return control orifices which meter fluid returned therefrom to a tank.
The input control orifices are formed at smaller radii than the return control orifices. Thus, output pressure between the first and second output ports is additively applied to either side of each of a plurality of effectively enlarged ridge sections which form the return control orifices. The product of the output pressure, the summed areas of the enlarged ridge sections, and their effective radii generates the hydraulic reaction torque.
Output pressure is coupled to a utilization device, such as a power cylinder, via flow restrictors. The flow restrictors are controlled orifice devices which can have a nominally linear flow resistance characteristic. For this reason, values of differential pressure applied to the utilization device are less than the output pressure. The reduction of output pressure is nominally proportional to fluid flow rate through the utilization device. This results in a controlled damping ratio and stable operation of systems incorporating the flow restrictors of the present invention.
Improved performance can be obtained from a servo system comprising the torque reaction valve by introducing an orifice in parallel with a double acting utilization device also comprised within the servo system. Fluid flow rate through the orifice improves system damping and results in an improved control characteristic wherein over-sensitive response to small involuntary control inputs are precluded.
A power steering system utilizing a torque reaction valve exhibits a substantially linear characteristic wherein steering wheel torque is proportional to steering load. However, some would prefer a first type of improved torque reaction valve wherein moderately increasing values of closed-loop transfer function are provided concomitantly with increasing steering loads. This would enable increased tactile feel of lighter steering loads together with achieving decreased values of steering wheel torque at higher steering loads.
A strongly non-linear characteristic commonly referred to as "center-point feel" (at low steering loads) is often featured on expensive European automobiles. "Center-point feel" comprises a delay of the application of power steering assist by a vehicular power steering system until some nominal value of torque has been applied to the vehicle's steering wheel (i.e., between 5 and 10 in.lbs. of torque). Ideally this is accomplished with input and output members of the power steering system's control valve locked together so that no rotational compliance is introduced thereby. When additional steering wheel torque is applied this condition is followed by application of power steering assist. The advantages of "center-point feel" include tactile feel of the road by the driver and avoidance of externally induced perturbations of the steering system (i.e., such as by street car tracks).
Thus, some others would prefer a second type of improved torque reaction valve wherein "center-point feel" is incorporated. And, still others would prefer a third type of improved torque reaction valve wherein both increasing values of closed-loop transfer function and "center-point feel" are incorporated. These three types of improved torque reaction valves are comprised in preferred embodiments of the present invention as described hereinbelow.